A drawback of the steam generator, in particular during heating up and cooling down is that the heat transfer coefficient between water and the vessel wall material, for instance steel, is substantially different from than the heat transfer coefficient between steam and the vessel wall material. The wall of the vessel next to the steam zone may therefore during start-up expand faster and/or at a earlier stage than the wall of the vessel next to the water zone due to the different rate of warming up. Similarly during turning down of the process, the wall of the vessel next to the steam zone may contract at a later stage than the wall of the vessel next to the water zone contract slower and/or at a later stage than the wall of the vessel next to the water zone.
Due to uneven expansion of the vessel wall, the vessel may bend over its longitudinal axis. This effect which is also referred to as ‘drum humping’, is undesired as it introduces undesired mechanical stresses within the vessel wall. In particular, when the process is often started-up and turned down, the mechanical stresses may lead to fatigue of the material and mechanical failure.